An electric generator transforms rotational energy into electrical energy according to generator action principles of a dynamoelectric machine. The turning torque supplied to a rotating rotor by a combustion or steam-driven turbine is converted to alternating current (AC) electricity, typically three-phase AC, in a stationary stator that surrounds the rotor. The generator is a mechanically massive and electrically complex structure, supplying output power up to 2,222 MVA at voltages up to 27 kilovolts. Electrical generators are the primary power producers in an electrical power system.
As shown in the cross-sectional view of FIG. 1, a conventional electric generator 10 comprises a substantially cylindrical rotor 12 supporting axial field windings or rotor windings 13. A direct current (DC) supplied to the rotor windings 13 produces a constant magnetic flux field that rotates with the rotating rotor within a stationary armature or stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas driven turbine (not shown in FIG. 1) for providing rotational energy to turn the rotor 12. The opposing end 16 is coupled to an exciter (not shown) for supplying the direct current to the rotor windings 13. An alternating current is generated in the stationary stator windings as the rotor's magnetic flux field crosses the stator windings. Rotor rotation subjects the rotor 12 and the rotor windings 13 to radial centrifugal forces that may result in radial distortion of these components.
The stator 14, a shell-like structure, encloses the rotor and comprises a core 17 further comprising a plurality of thin, high-permeability circumferential slotted laminations 17A placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils are wound within the inwardly directed slots. The AC electricity induced in the stator windings by action of the rotor's rotating magnetic field flows to terminals 19 mounted on the generator frame for connection to an external electrical load. Three-phase alternating current is produced by a generator comprising three independent stator windings spaced at 120° around the stator shell. Single-phase alternating current is supplied by a stator having a single stator winding.
The rotor 12 and the stator 14 are enclosed within a frame 20. Each rotor end comprises a bearing journal (not shown) for cooperating with bearings 30 attached to the frame 20.
A generator cooling system removes heat produced by current flow through the generator conductors, including the direct current flow through the rotor windings 13 and the alternating current induced in the stator coils. Additional heat sources include mechanical losses, such as windage caused by the spinning rotor, and friction at the bearings 30. The rotor 12 carries a blower 32 for forcing cooling fluid through the generator elements. Coolers 36 receive and cool the cooling fluid to release the heat absorbed from the generator components. The cooling fluid is then recirculated back through the generator components.
To increase generator output and efficiency and reduce generator size and weight, conventional copper rotor windings are replaced by superconducting windings (filaments) that exhibit effectively no resistance to current flow when maintained below the material's critical temperature (TC). Superconductivity is a phenomenon observed in several metals and ceramic materials when the material is cooled to temperatures ranging from near absolute zero (0° K or −273° C.) to a liquid nitrogen temperature of about 77° K or −196° C. The critical temperature for aluminum is about 1.19° K and for YBa2Cu3O7 (yttrium-barium-copper-oxide) is about 90° K. Yttrium-barium-copper-oxide (one example of a high temperature superconducting (HTS) material) is commonly used for the rotor windings of a superconducting generator.
Since the superconducting materials exhibit substantially no electrical resistance when maintained at or below their critical temperature, these materials can carry a substantial electric current for a long duration with insignificant energy losses. To maintain the superconducting conductors at or below their critical temperature, coolant flow paths carrying coolant supplied from a cryogenic cooler are disposed adjacent or proximate the windings. Typical coolants comprise liquid helium, liquid nitrogen and liquid neon.
Disadvantageously, the HTS rotor windings are sensitive to mechanical bending and tensile stresses that can cause premature degradation and winding failure (e.g., an open circuit). For example, bends formed in the HTS rotor windings to circumscribe the cylindrical rotor core induce winding stresses. Normal rotor torque, transient fault condition torques and over-speed forces induce additional stress forces in the rotor windings. These over-speed and fault conditions substantially increase the centrifugal force loads on the rotor coil windings beyond the loads experienced during normal operating conditions.
The co-pending commonly-owned application entitled Superconducting Coil Support Structures Ser. No. 11/529,373 describes and claims HTS winding support structures that support the windings against these loads. This application is incorporated by reference herein. The support structures also limit heat transfer from the “warm” (i.e., approximately room temperature) rotor core to the “cold” (i.e., cryogenically cooled) HTS windings. In addition to conductive thermal paths in the support elements, it is desired to maintain the HTS rotor windings in a vacuum condition to limit radiative heat transfer from the rotor core to the superconducting windings.
AC electricity available at the stator terminals is supplied to an electrical power grid through a transmission and distribution system. Grid fault currents, e.g. caused by a lightning-induced current spike on the grid, are coupled to the stator through the intervening transmission and distribution lines. The grid fault currents generate a stator fault current and an attendant strong transient magnetic flux that is magnetically coupled to the HTS rotor windings. This flux can generate a significant torque on the rotor core and the HTS winding, potentially damaging the HTS winding and its support structures. The transient magnetic fields can also be caused by system or internal short circuits, transmission switching operations, synchronizing operations, transient voltages on the transmission system and loss of synchronism between the generator and the grid. In addition to the undesired mechanical forces produced by these transient torques, any magnetic field coupled into the rotor windings causes undesired alternating current (AC) losses in the HTS conductors.
Although rotor winding support structures can be designed to allow the HTS conductors to withstand the additional torque introduced by these transient magnetic fields, such support structures increase the support mass and may introduce additional undesired thermal paths between the warm rotor core and the cold HTS windings.
Typically however, the rotor windings are shielded to prevent transient magnetic fields from reaching the rotor HTS windings. An electromagnetic shield, comprising copper or aluminum for example, encloses the HTS rotor windings to prevent magnetic flux from coupling to the rotor, thereby avoiding the consequent torques induced on the HTS windings. The shield is also referred to as a non-magnetic shield since it is constructed from non-magnetic material.
For relatively small electric generators the shield comprises a thin tubular or cylindrical structure surrounding the rotor core and the HTS windings and attached to the rotor core end faces. However, it is a substantial challenge to manufacture, assemble and balance a large and continuous cylindrical shield structure with the precision and tolerances required for a large electrical generator. According to one embodiment, a tubular shield having a relatively thin wall surface is supported by the rotor shaft with a tight clearance between the rotor and the shield. Gravity loading deforms the thin tube into an elliptical shape and interface contact is made at the top and bottom surfaces of the rotor shaft. Further, the considerable rotor weight tends to cause rotor sag. These effects lead to fretting damage due to relative motion (albeit a small displacement) at the interface of the rotor core and the non-magnetic shield. Alternatively, the tube shield has relatively thick wall surface with a larger gap between the shield and the rotor. Little or no fretting damage occurs in this configuration, but the shield must be sufficiently thick to support its own weight.
The rotor core and the surrounding non-magnetic shield independently vibrate at a different resonant frequency with a different vibration pattern. These effects create additional dynamic loads on the rotor core and the HTS windings. The cumulative effect of the interface contact forces and the vibration forces create extremely high stresses on the rotating non-magnetic shield.
If the rotor core and the HTS rotor windings are enclosed in a vacuum vessel (comprising stainless steel for example) additional design difficulties arise. If the shield and the vacuum vessel are both cylindrical with the vacuum vessel nested within the shield they are preferably joined to maintain the vacuum condition. Joining the dissimilar metals of the vacuum vessel and the shield is problematic. Further, the disadvantages associated with the large generator shield discussed above are exacerbated by the addition of the vacuum vessel.
It is known by those skilled in the art that the rotor must be balanced to minimize undesired rotor torques. During the balancing process balancing weights are added to the rotor body at various locations along its axial length to balance the rotor at its operating speed. Effective balancing requires access to the entire rotor body surface to permit placement of the balancing weights as desired to effect a balanced condition. A shield that covers the entire rotor requires performing the balancing operation prior to placement of the shield over the rotor. But such a process increases production cycle time and process costs. Also, this pre-shield installation balancing operation is conducted with the rotor at ambient temperature, but the rotor operates at cryogenic temperatures. Undoubtedly, the lower temperature affects the rotor's balance. Thus it is preferable to balance the rotor under cryogenic operating conditions with access to the entire rotor surface to place balance weights as required.